Three filling pipes R, S and T together can fill an empty tank in 2 ho

How did you solve the fraction? I didn't get that part.

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First of all, this is a problem solving question so it is bound to have a unique solution.
All three together take 2 hrs so their combines rate is 1/2 tank per hour.
But R is opened alone separately and S and T are opened together but independently of R. The time taken in this case is 4 hrs, twice of time taken before. This happens when the rate of work of 2 individuals/groups is equal. When they work together, time taken is half of time taken when they work independently.
So rate of R must be equal to rate of S and T together.
If rate of T is T, rate of S is 4T (four times rate of T). Then rate of R is 5T (same as combined rate of S and T).

Combined rate of all three = T + 4T + 5T = 10T = 1/2 tank per hour
Rate of T = 1/20th tank per hour

Time taken by T alone is 20 hrs.

Answer (C)

Note that it doesn't matter what x and y are. They could be 1 and 3 respectively or 1.5 and 2.5 or 2.1 and 1.9 etc. Since both the rates are same ('Rate of R alone' = 'Rate of S+T') time taken will be 4 hrs only. Whether we make only R work or S and T work, it has the same effect.

For more on this, check out the work-rate module here: https://anglesandarguments.com/study-module

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